This invention relates to a modem generally, and more particularly to a modem with a circuit for extending its dynamic range.
At present, data communication relies almost entirely on the network of telephone lines. The frequency bandwidth, attenuation distortion, delay distortion, signal-to-noise ratio, and other characteristics of the telephone lines are optimized for carrying voice conversations and are not suited for carrying data signals. Accordingly, in data communication via telephone lines, the transmitting terminal must convert the data signal by using it to modulate a carrier signal compatible with the line characteristics, and the receiving terminal must demodulate the received signal to recreate the original data signal. These modulating and demodulating functions are usually performed by a modem, which is a device inserted between the data terminal equipment and the telephone line. In a system that transmits a variety of types of data or communicates with a variety of terminals, it is necessary for the modem to be able to operate under a wide range of received signal levels; that is, the modem must have a wide dynamic range.
A problem of prior-art modems is that the dynamic range of the modem is at most equal to the dynamic range of its demodulating circuit. To widen the dynamic range of the modem it is therefore necessary to widen the dynamic range of the demodulating circuit, but this is difficult and expensive; that is, it requires a complex circuit configuration making use of advanced technology.